Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple-access technologies include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, single-carrier frequency divisional multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems.
These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example of an emerging telecommunication standard is Long Term Evolution (LTE). LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by the Third Generation Partnership Project (3GPP). It is designed to better support mobile broadband Internet access by improving spectral efficiency, lower costs, improve services, make use of new spectrum, and better integrate with other open standards using OFDMA on the downlink (DL), SC-FDMA on the uplink (UL), and multiple-input multiple-output (MIMO) antenna technology. However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in LTE technology. In various aspects of the present disclosure, these improvements should be applicable to other multi-access technologies and the telecommunication standards that employ these technologies.
Within modern cellular telecommunications systems, heterogeneous networks are recently gaining significant attention. In a heterogeneous network, base stations of different sizes and power levels operate together to provide more complete coverage, particularly in urban areas where the signal from high-power base stations, frequently called macrocells, fails to reach certain areas such as indoors. Thus, heterogeneous networks may include one or more types of low-power base stations, which are frequently called microcells, picocells, and femtocells to improve coverage in the indoor locations.
In particular, femtocells utilize existing infrastructure deployed for Internet communication as a network backhaul for wired communication with the cellular network. In this way, for example, a user may deploy a femtocell in an indoor location where improved cellular coverage is desired by connecting a femtocell to the Internet through a cable or DSL modem.
LTE can support both asynchronous and synchronous configurations. When LTE is deployed in a synchronous configuration, time and frequency synchronization is critical for femtocell (called a Home eNode B or HeNB in LTE standards) operation. Typically, a femtocell can derive its clock time from the network backhaul (e.g., Network Time Protocol (NTP), Precision Time Protocol (PTP), or other variants), from a “network listen” circuit (e.g., an RF circuit that can receive and interpret information from the air channel, transmitted from nearby macrocells), and/or by utilizing GPS-based methods.
However, meeting synchronization requirements is especially challenging for femtocells. Femtocells are generally low cost, unplanned deployments in indoor environments with limited operator control. Additionally, there may be loose coordination with macrocells and unreliable backhaul connectivity. Typically, femtocells use a low cost or poor quality oscillator that requires frequent time or frequency offset corrections. Furthermore, reception quality of GPS and macrocell signals can change significantly based on femtocell placement and location, and backhaul based synchronization may not meet time accuracy requirements.